The present invention generally relates to delivery systems for cryogenic fluids and, more particularly, to a delivery system that supplies high pressure cryogenic fluids from a low pressure cryogenic liquid container without the use of pumps or compressors.
Cryogenic liquids, that is, liquids having a boiling point generally below -150.degree. F. at atmospheric pressure, are used in a variety of applications. Many of these applications require that the cryogen be supplied as a high pressure gas. For example, high pressure nitrogen and argon gases are required for laser welding and metal powder production while high pressure nitrogen, oxygen and argon gases are required for laser cutting.
These cryogens are stored as liquids, however, because one volume of liquid produces many volumes of gas (600-900 volumes of gas per one volume of liquid) when the liquid is allowed to vaporize (boil) and warm to ambient temperature. To store an equivalent amount of gas requires that the gas be stored at a very high pressure. This requires a container that is larger and much heavier than if the cryogen is stored as a liquid. It also normally requires expensive, high maintenance compressors or pumps to increase the pressure to the required high level. Examples of other cryogens stored and transported as liquids, but used as gases, include hydrogen, helium and liquified natural gas (mostly methane). Carbon dioxide is not generally recognized as a cryogen, but is also stored as a cold liquid in highly insulated containers and used as a gas.
Many cryogenic products of the type mentioned above are used in applications requiring fluids at pressures between 100 psi and 400 psi. Existing systems, such as the VCS system manufactured by MVE, Inc., utilize a bulk cryogenic storage tank with an operating pressure equivalent to the pressure required by the application. The pressure within the storage tank is increased with a conventional pressure building system. More specifically, cryogenic liquid from within the storage tank is fed to a heat exchanger where it is heated by the ambient air. The vapor thereby created is returned to the top of the storage tank so that the pressure within the tank is increased. The tank and its contents are at a higher elevation than the pressure building heat exchanger so that the cryogenic liquid is gravity fed to the latter through a regulating valve. When the desired pressure is reached within the bulk storage tank, the regulating valve is closed thus stopping the flow of cryogenic liquid to the heat exchanger. Cryogenic liquid from within the bulk tank is then delivered to the application at the desired pressure to be used as liquid, or is vaporized in another heat exchanger if gas is required.
While this type of system works well, cryogenic storage tanks that are able to hold pressures over 250 psi are expensive when compared to lower pressure cryogenic tanks. Furthermore, such systems are limited to providing cryogenic fluids at a pressure of 400 psi or less. This is because the delivery systems (that are on a transport such as a truck or railroad car) that refill the bulk cryogenic storage tank feature pumps that cannot deliver product to a bulk storage tank that is at a pressure greater than 400 psi. As a result, if the pressure in the bulk cryogenic storage tank is increased to a level above 400 psi, it must be vented prior to being refilled. Such venting is wasteful and may be unsafe or detrimental to the environment.
Accordingly, an object of the invention is to provide a cryogenic delivery system that can utilize existing low pressure cryogenic storage containers while supplying cryogenic fluids at higher pressures. Another object of the invention is to provide a high pressure cryogenic delivery system that does not require venting.
U.S. Pat. Nos. 5,421,160 and 5,537,824 to Gustafson disclose fueling systems for natural gas powered vehicles that use a bulk cryogenic storage container for storing a large quantity of liquid natural gas (LNG) at a low pressure. The LNG is delivered to two relatively small volume fuel transfer tanks wherein the pressure and temperature of the LNG may be raised or lowered as dictated by the needs of the application. This is accomplished by delivering high pressure natural gas vapor to the fuel transfer tanks from a high pressure bank consisting of one or more heat exchangers, a compressor and a number of small volume, high pressure storage tanks. LNG flows from the bulk container to the heat exchanger of the bank where it is vaporized. The vapor thus produced is compressed by the compressor to a high pressure and is then stored in the small high pressure tanks. The compressor may also be used to reduce undesirable pressure buildup in the bulk container by removing vapor from its head space. This avoids the need for venting the bulk tank.
While this system also works well, it requires the use of high pressure compressors or pumps to produce the high pressure gas and to control the pressure in the bulk tank. Such compressors and pumps are expensive to purchase and maintain. Furthermore, the use of high pressure compressors or pumps increase the power requirements of the system while decreasing its reliability. Accordingly, another object of the invention is to provide a cryogenic delivery system that can increase the pressure of cryogenic liquids and control the pressure in the bulk tank without the need for high pressure pumps or compressors.